The invention relates to an apparatus for encoding a wideband digital signal, the apparatus including:
input means for receiving the wideband digital signal, and
splitter means for splitting the wideband digital signal into a number of M sub signals, M being an integer larger than one,
formatting means for assembling digital information including said sub signals into a digital output signal having a format suitable for transmission or storage, to a method of encoding, to control signal generator means that can be used in the apparatus, to a decoding apparatus and to a decoding method.
An encoding apparatus as defined in the opening paragraph is well known in the form of a subband encoder or a transform encoder. Reference is made in this respect to EPA-402.973, to which U.S. Pat. No. 5,323,396 corresponds. The above citation and the citations below are all hereby incorporated in whole by reference.
The invention has for its object to provide an improved encoding apparatus. The apparatus in accordance with the invention as defined in the foregoing is characterized in that the apparatus further comprises delay means coupled between the input means and the splitter means, for realizing a specific delay of the wideband digital signal.
The invention is based on the following recognition. Investigations carried out on the quality of a wideband digital signal that has been repeatedly encoded and decoded, have resulted to the conclusion that such repeated encoding and decoding may lead to some signal degradation. It was recognized that one of the causes of this signal degradation was the difference in the timing in a decoder and a subsequent encoder. More specifically, it was recognized that, in a system where a decoding apparatus is coupled to a subsequent encoding apparatus, the signal delay as a result of the series connection of synthesis filter means present in the decoding apparatus and the splitter means present in the encoding apparatus and their interconnection is an important factor. This, in the sense that this delay should be equal to MT, or equal to an integer multiple of M.T, where T is the sampling period of the wideband digital signal, in order to have the lowest signal degradation.
If this delay is however not equal to MT or to an integer multiple of M.T, the additional delay means as claimed should be inserted between the output of the synthesis filter means and the input of the splitter means, the delay of the additional delay means being such that the total delay, inclusive the delay of the delay means, satisfies the above requirement.
As a result, a delay means having a fixed delay can be included in the input means of the encoding apparatus or in the output means of the decoding apparatus, assuming that the splitter means in any encoding apparatus have all the same delay and that the synthesis filter means in any decoding apparatus have the same delay.
If wideband digital signals have been obtained upon decoding in a decoding apparatus having splitting filter means of an unknown delay, it will be necessary to incorporate a variable delay in the encoding apparatus in order to satisfy the requirement that the total delay equals M.T or equals an integer multiple of M.T.